1. Field of the Invention
The present invention concerns a switching element with multiple operating modes and a switching network incorporating a plurality of such switching elements, in particular for switching asynchronous time-division multiplex packets, although this latter application is described by way of example only, the invention being of wider application.
2. Description of the Prior art
Packets are units of digital information including a label from which the destination of the packet can be identified and call data. Packets relating to a plurality of calls are transmitted over the same transmission link in any order.
For switching asynchronous time-division multiplex packets a switching element comprises inputs, outputs, selective retransmission means for retransmitting a packet received on one input to one or more outputs on the basis of routing data associated with the packet and, usually, retransmission control means which determine the mode of operation of said retransmission means.
A switching network using these switching elements comprises input ports, output ports and switching elements between the input ports and the output ports. The switching elements are arranged in one or more stages interconnected by links. The input ports of the switching network are associated with the inputs of switching elements of a first stage and the output ports are associated with the outputs of switching elements of a final stage. The outputs of the switching elements of said first stage are connected by links, possibly through intermediate switching element stages, to the inputs of the switching elements of said final stage.
The function of the network in packet switching is to route packets selectively between an input port of the network and at least one output port. To be more precise, there will be provision at least for so-called point-to-point routing from an input port to an output port and for so-called point-to-multipoint routing from an input port to a plurality of output ports.
French patent applications Nos. 8716475 and 8716476 describe a switching element that can be used to implement a switching network of this kind.
This switching element receives packets on its inputs and retransmits them selectively to its outputs. Two embodiments of the switching element are described. The first, referred to hereinafter as the direct routing embodiment, is for the situation in which the receive packets include a destination indication in a label. The destination indication is decoded in the switching element to obtain the identifier of at least one output to which the packet must be retransmitted, which identifier is then used directly to control retransmission of the packet to each output so designated. As the packet has to pass through other switching elements in a switching network, the destination indication is in a number of parts which are used successively. The second embodiment, referred to hereinafter as the translated routing embodiment, is for the situation in which the label includes a reference number called the virtual circuit number. In each switching element the reference number is used to address a translation memory which supplies in response the identifier of at least one output to which the packet must be retransmitted.
Note that with either type of routing the switching element itself is in no way involved in the designation of the outputs to which the packet is retransmitted. A unit external to the switching network must supply the routing data identifying the packet path or paths in the switching network, in other words either the destination indication included in the packet label in the first kind of switching element or the data written into the translation memories of the switching elements in the case of the second kind of switching element.
The direct routing switching element is particularly attractive because of its simplicity: it is sufficient to decode the destination indication to deduce the designation of the output to which a packet must be retransmitted in the case of point-to-point routing or the designations of the outputs to which the packet must be retransmitted in the case of point-to-multipoint routing. Its disadvantage is that it is not usable in practice if the switching network must be able to broadcast a packet arriving on any input port to any plurality of output ports. It is then necessary to make provision for the broadcast function to be implemented in any switching element of the switching network; in this switching element a received packet is retransmitted identically to a plurality of outputs. A plurality of switching elements of the next stage in the switching network will receive these identical packets. They will therefore have no means of routing the packet to any specified output as designated by a specific destination indication of the packet label. This type of switching element can therefore be used only in the final stage of a switching network comprising multiple switching element stages, and then provides only for broadcasting to the outputs of a single switching element, which does not meet the stated requirement.
The translated routing switching element does meet the requirement stated above with regard to broadcasting to any plurality of output ports but has its own disadvantages. The translation memory of each switching element must receive the output identifiers for each of the reference numbers likely to be routed by the switching element. As the reference number must not include too many digits, for reasons of transmission efficiency, the number of reference numbers available is limited which means that they have to be used again, which makes it necessary to provide new output identifiers in the translation memories of the switching elements each time and to erase them later and requires costly transmission resources to carry this out. There are even switching network configurations in which the first stages cross-connect the traffic so that a packet can be routed to any output of these first stages before being routed to its destination. The result of all this is that the routing of any packet must be preceded by the supply of routing data to practically all the switching elements of the switching network. This may be impracticable if there are a large number of switching elements.
The present invention is directed to solving this problem by proposing a switching element offering the advantages of both kinds of switching element described above but which largely circumvents the previously described disadvantages, together with a switching network comprising, at least in part, switching elements of this kind and exploiting their advantageous characteristics.
On a more general level, the switching element in accordance with the invention finds an application whenever it is necessary to route information by switching it, using at least the two routing methods described above, irrespective of the form of the information to be routed. The word "packet" as used in the present document is not to be understood as referring to a specific presentation of the switched information, but to the contrary has the widest possible meaning.